Method of making electrical heating elements



Dec. 5, 1967 3,355,802

METHOD OF MAKING ELECTRICAL HEATING ELEMENTS Fig. 3.

Filed Jan. 5, 1966 .J. w. RUTTER ETAL "I .,e 6% 4 a mwa/ w n n M W d nr u ow d$ United States Patent 3,355,802 METHOD 01F MAKING ELECTRICAL HEATING ELEMENTS John W. Rutter, Schenectady, N.Y., and Edward M. Clausen, Warrensvilie Heights, Ghio, assignors to General Eiectric Company, a corporation of New York Filed Jan. 3, 1966, Ser. No. 513,233 4 Claims. (Cl. 29-614) This invention relates to electric resistance heating elements and is more particularly concerned with a new method of making such devices through the use of spheroidal fused magnesium oxide powder.

Heating elements of the type comprising an inner, electrically resistive conductor, a surrounding layer of magnesia, electrical insulation and an outermost protective jacket are widely used in many industrial heating devices as well as in devices such as household ovens. This type of heating element is much more durable than, for example, exposed resistance wire. Structurally, it usally includes: (l) a coiled resistance wire composed of alloys such as those made up of -16 percent chromium, 59-62 percent nickel, about 24 percent iron and 0.1 percent carbon; (2) compacted magnesia powder containing minor amounts of impurities surrounding the resistance coil as an insulator; and (3) an outer protective jacket normally constructed of metals ranging from lead to high-temperature alloys.

While the service life of these elements is generally good, the manufacture of them is not a straight-forward and easy as desired. In particular, the forming of the magnesia powder compact around the conductor and the subsequent shaping operations involved in producing units of desired geometry require special care.

We have found that these compacting and shaping operations can be simplified and expedited without any loss of eificiency and without adversely affecting the quality or the reliability or the service life of the finished electrical heating element or unit. This result is achieved through the use of high purity fused magnesium oxide powder which has been specially prepared so that it is made up only or principally of spheroidal particles. Such magnesium oxide powder has flow characteristics which are superior to those of the fused magnesium oxide powders heretofore used in the production of electrical resistance heating elements. This spheroidal particle powder also enables the achievement of higher packing densities in the manufacture of these heating units. Still another advantage of this invention method is the fact that one and possibly two of the steps involved in the present commercial process of producing fused magnesium oxide powder for electrical heating elements may be eliminated with substantial saving in cost and without incurring any significant off-setting disadvantage.

In its broadest aspect, the present invention centers in the concept of using spherical or spheroidal particle fused magnesium oxide powder in the manufacture of electrical heating elements and units. Thus, the present novel method comprises the steps of producing spheroidal particles of magnesium oxide by creating a radio frequency plasma in a stream of gas having at least 0.1 atmospheric pressure of oxygen, then introducing finely divided magnesium oxide into the gas at the upper end of the plasma, allowing the finely divided magnesia to fall through the plasma and quenching the resulting particles in a fluid, packing the quenched fused spheroidal particles around an electrically resistive conductor, and enclosing the resulting assembly in a jacket.

More in detail, in accordance with the present invention, previously calcined magnesium oxide is ground and then passed into the plasma of a radio frequency induction torch, as disclosed in article by T. B. Reed, Growth of Refractory Crystals Usin the Induction Plasma Torch, lournal of Applied Physics, vol. 32, No. 12, pages 2534-35 (1961). Torches of this type comprise a stream of gas that is subjected to a radio frequency which causes ionization of the gas to produce a temperature of about 15,000 K. Whereas, argon is usually the gas used in these torches, other gases, such as oxygen, may be added to percent of the total gas stream, to bring the partial pressure of the oxygen in the torch above the level required to prevent the reduction of magnesium oxide.

The effect produced by the plasma on the magnesium oxide is influenced by the following factors:

(I) The state of aggregation of the particle.

(2) The level of the power output of the inductor.

(3) The plasma gas composition and flow rate. For example, the increase of the oxygen content of the plasma gas results in greater adsorption of power from the radio frequency field due to dissociation of the molecule.

(4) Dwell time of the particle in the plasma.

(5) Particle size. The particle size is distinct fro-m the aggregation. A small particle may be vaporized while a particle having the same mass but more porous is only surface melted. For best results, the particles must be sized before the material is passed through the torch. Different plasma conditions were required for satisfactory processing of a 60+100 mesh powder fraction than for a 100l+2(i0 fraction.

(6) Velocity of injection of powder into the plasma. At

low powder feed gas flow velocities, much of the powder does not enter the plasma but is thrown back as a result of the violent expansion which occurs when the plasma gas is raised to a very high temperature. At high powder feed gas flow rates, nearly all of the powder is injected into the plasma but the dwell time is decreased along with the melting. The plasma condition should be chosen so that it produces satisfactory results and maximizes the amount of powder that enters the plasma. The powder feed rate is generally 10 to 100 gms. per hour.

A. one-inch torch usually requires frequencies in the range of 3 to 10 me. The ionized argon, being a conductor, is induction heated. This produces a plasma about 10 centimeters long with the hottest zone at the coil of about 1 centimeter.

Magnesium oxide, which has been previously calcined, is ground to a powder within the range of 100 to +260 mesh and is injected into the plasma from a side tube in a stream of gas of sutficient velocity to force it into the plasma. The volume or" gas passing through a 2 mm. feed tube is between 3 to 5 cubic feet per hour. This provides a dwell time of the particles in the plasma of about 10- seconds, and in the hottest zone to about 10* seconds. It is then allowed to drop through the plasma and collected in a water bath. The gas flowing through the plasma torch must contain at least 0.1 atmosphere partial pressure of oxygen. It has been found that the preferred gas contains 2 parts argon and 1 part oxygen.

The magnesium oxide recovered from the water bath is spherical and has a frosty appearance due to the thermal shock received in the quenching step.

It may be desirable in some cases to extend the length of the radio frequency coil or to add a controlled temperature zone below the radio frequency coil and above the water bath in order to attain more complete spheroidization and better physical properties of the magnesium oxide particles. It is obvious other fluids or liquids than water may be used to quickly cool the particles.

We have produced fused magnesium oxide in accordance with this invention for use in making an electrical resistance heating element by feeding 60 +100 mesh magnesia powder into a plasma. This piasma was produced through the use of a one-inch diameter torch having a 20 kw. heater and frequency of approximately 7 mc. The plasma gas was 75 percent oxygen and 25 percent argon (by volume) and the magnesia powder was injected into the plasma at the rate of 45 grams per hour. In similar operations, we have produced fused magnesium oxide using 100 +200 mesh magnesia powder and injecting it at a powder feed rate of 66 grams per hour in one case and 72 grams per hour in another. In the last instance, the plasma gas was 62 percent argon and 38 percent oxygen (by volume) While in the other case it was 75 percent argon and 25 percent oxygen (by volume). All the other conditions were as stated in the first operation above.

A special feature of this invention process is that the product does not contain contaminants resulting from the fusing of the oxide. It may therefore find extensive use in the preparation of electrical parts, where contamination by metals or carbon is particularly undesirable. Such a use is the preparation of electrical resistance heating units.

Referring to the drawings accompanying and forming a part of this specification:

FIG. 1 is an enlarged side elevation, partly broken away, of the type of heating element with which this invention is concerned;

FIG. 2 is a drawing made from a photomicrograph (at 100 diameters) of a sample of the fused magnesia powder presently used in the commercial production of electrical resistance heating elements; and

FIG. 3 is a drawing like that of FIG. 2 of a sample of the spheroidal particle fused magnesia (at 100 diameters) made and used in accordance with this invention in the production of electrical resistance heating elements.

Referring to FIG. 1, the numeral indicates the type of resistance heating device in which the insulation of this invention is most useful. Element 10 is constructed of an innermost, electricaily resistive conductor 11 through which electricity flows. Normally, these conductors 11 are constructed in a spiral or helical form, as indicated, and are composed of high resistance alloys. For example, suitable alloys include those such as Nichrome, which are high resistance alloys, composed of from 15 to 16 percent chromium, 59 to 62 percent nickel, about 24 percent iron and about 0.1 percent carbon.

Surrounding the conductor 11 is a quantity of particulate magnesia 12-which is packed tightly about conductor 11, this compaction usually being performed by a swaging operation although rolling is used occasionally. The magnesia used is the spheroidal particle powder described above prepared in accordance with the procedure stated in detail herein. The structure of the heating element is completed by an outermost metallic jacket 13 which serves to protect the elements 11 and 12 from breakage. This jacket will be constructed of some metal which is sufficiently heat-resistant to withstand the designed Operating temperature of the heating device. Thus, for relatively low temperature applications, a metal such as lead can be used, whereas for higher temperature applications, a high alloy metal such as Inconel can be used. Inconel is a high nickel-chromium iron alloy which, in the wrought form, contains 79.5 percent nickel, 13 percent chromium, 6.5 percent iron, 0.25 percent manganese, 0.25 percent silicon, 0.8 percent carbon and 0.20 percent copper.

As indicated above, the fused magnesia powder presently used in the commercial production of electrical resistance heating devices of the FIG. 1 type is illustrated in the drawing of FIG. 2. This drawing actually is a sketch made from a photomicrograph of a typical sample of this fused powder material at a magnification of diameters. As a result of the preparation and processing of this powder, it is composed of irregularly-shaped particles 16 of widely different sizes.

The particles of fused magnesia illustrated in FIG. 3 by contrast are regular and spheroidal in shape although, again, of widely different size. These spheroidal particles 18, as also indicated above, are produced in accordance with this invention and are useful in the production of electrical heating elements as illustrated in FIG. 1, with the advantages stated above. PEG. 3, like FIG. 2, is a sketch made from a photomicrograph of a typical spheroidal fused magnesia powder prepared in accordance with this invention, the photomicrograph being taken at 100 diameters as in the case of FIG. 2.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A method of making an electrical heating element which comprises producing finely divided spheroidal particles of magnesium oxide by producing a radio frequency plasma in a stream of gas having at least 0.1 atmospheric pressure of oxygen, then introducing finely divided magnesium oxide into said gas at the upper end of said plasma, allowing said particles to fall through said plasma and quenching the same in a fluid, packing the resulting spheroidal fused magnesia particles around an electrically resistive conductor, and enclosing the resulting magnesia powder compact-conductor assembly in a jacket.

2. A method as set forth in claim 1, wherein the finelydivided magnesium oxide is within the range of 60 to +100 mesh.

3. A method as set forth in claim 1, wherein the gas stream is composed of 2 parts argon and 1 part oxygen.

4. A method as set forth in claim 1, wherein the magnesium oxide is quenched in water.

References Qited UNITED STATES PATENTS 2,846,537 8/1958 Thornhill 29615 X 3,007,236 11/1961 Barnes 29617 3,050,833 8/1962 Schwing 29614 3,065,436 11/1962 Kayko et al. 296l3 X 3,330,034 7/1967 Price 29-615 X JOHN F. CAMPBELL, Primary Examiner.

J. L. CLINE, Assistant Examiner. 

1. A METHOD OF MAKING AN ELECTRICAL HEATING ELEMENT WHICH COMPRISES PRODUCING FINELY DIVIDED SPHEROIDAL PARTICLES OF MAGNESIUM OXIDE BY PRODUCING A RADIO FREQUENCY PLASMA IN A STREAM OF GAS HAVING AT LEAST 0.1 ATMOSPHERIC PRESSURE OF OXYGEN, THEN INTRODUCING FINELY DIVIDED MAGNESIUM OXIDE INTO SAID GAS AT THE UPPER END OF SAID PLASMA, ALLOWING SAID PARTICLES TO FALL THROUGH SAID PLASMA AND QUENCHING THE SAME IN A FLUID, PACKING THE RESULTING SPHEROIDAL FUSED MAGNESIA PARTICLES AROUND AN ELECTRICALLY RESISTIVE CONDUCTOR, AND ENCLOSING THE RESULTING MAGNESIA POWDER COMPACT-CONDUCTOR ASSEMBLY IN A JACKET. 